blk-mq: introduce new queue attribute async_depth

Changes in v7:
 - add review tag;
 - rebase with for-7.0/block;
Changes in v6:
 - split patch 3 to factor out a helper;
 - improve some wording;
Changes in v5:
 - just resend with new email server;
Changes in v4:
 - add review tag for the last patch 3;
 - rebase with my new email address, patches are not changed;
Changes in v3:
 - use guard()/scope_guard() in patch 3;
 - add review tag other than patch 3;
Changes in v2:
 - keep limit_depth() method for kyber and mq-deadline in patch 3;
 - add description about sysfs api change for kyber and mq-deadline;
 - improve documentation in patch 7;
 - add review tag for patch 1;

Background and motivation:

At first, we test a performance regression from 5.10 to 6.6 in
downstream kernel(described in patch 5), the regression is related to
async_depth in mq-dealine.

While trying to fix this regression, Bart suggests add a new attribute
to request_queue, and I think this is a good idea because all elevators
have similar logical, however only mq-deadline allow user to configure
async_depth.

patch 1-4 add new queue attribute async_depth;
patch 5 convert kyber to use request_queue->async_depth;
patch 6 covnert mq-dedaline to use request_queue->async_depth, also the
performance regression will be fixed;
patch 7 convert bfq to use request_queue->async_depth;

Yu Kuai (8):
  block: convert nr_requests to unsigned int
  blk-mq-sched: unify elevators checking for async requests
  blk-mq: factor out a helper blk_mq_limit_depth()
  blk-mq: add a new queue sysfs attribute async_depth
  kyber: covert to use request_queue->async_depth
  mq-deadline: covert to use request_queue->async_depth
  block, bfq: convert to use request_queue->async_depth
  blk-mq: add documentation for new queue attribute async_dpeth

 Documentation/ABI/stable/sysfs-block | 34 ++++++++++++++
 block/bfq-iosched.c                  | 45 ++++++++----------
 block/blk-core.c                     |  1 +
 block/blk-mq-sched.h                 |  5 ++
 block/blk-mq.c                       | 68 ++++++++++++++++++----------
 block/blk-sysfs.c                    | 42 +++++++++++++++++
 block/elevator.c                     |  1 +
 block/kyber-iosched.c                | 33 ++------------
 block/mq-deadline.c                  | 39 ++--------------
 include/linux/blkdev.h               |  3 +-
 10 files changed, 156 insertions(+), 115 deletions(-)

-- 
2.51.0